Almost everything written about peptide vials is about what is inside them. The closure gets a sentence, usually that it is a rubber stopper. It is worth more than that, because the closure is the half of the container that gets handled, pierced and relied on, and it is therefore the half that fails.
Two components doing two jobs
The glass tube is a container. Its job is to hold the material and to be chemically inert, which borosilicate glass is to a close approximation.
The closure is a barrier. Its job is to keep water vapour, oxygen and microorganisms out, to hold whatever atmosphere was sealed in, and then to let a needle through and close behind it. Those requirements partly conflict, which is why the closure is an engineered component rather than a plug.
Look at a sealed vial from the top and you are seeing three things stacked. An elastomeric stopper sits in the neck. An aluminium collar is crimped around the neck and over the stopper's flange, holding it in compression. A plastic cap, which flips or tears off, covers the stopper's exposed face to keep it clean until first use. Removing that cap does not open the vial; the crimped collar and the stopper remain, and the vial is still sealed.
The stopper
The disc under the collar is an elastomeric closure, and for lyophilized material it is typically a butyl rubber. Butyl is used because its permeability to water vapour is very low, which matters enormously for a dry cake that will take up water from anywhere it can. It also recovers its shape after being pierced, which is the reseal behaviour the whole design depends on.
Stoppers are often coated or laminated on the product-facing surface, typically with a fluoropolymer film, which reduces both the extraction of rubber components into the solution and the adsorption of the solution's contents onto the rubber. For a peptide at low concentration the second of those is not trivial, and it sits alongside the surface-loss problem described in why peptides stick to plastic.
The colour is the detail most often misread. Grey, red, blue and white stoppers differ by formulation, by the filler and curing system the manufacturer used, and the colour is a marker for that formulation. It is not a quality grade and a grey stopper is not better or worse than a red one. Any inference from colour alone is unsupported.
There is a published standard for what these closures must do, including a defined test for coring, and the relevant point here is modest: such requirements exist, and we do not hold closure test data for this catalogue's vials, so nothing in this post should be read as a claim about them.
The headspace
At the end of a lyophilization cycle the vials are inside the chamber under vacuum, and the stoppers are seated while they are still in there. Most processes then admit an inert gas to bring the pressure part of the way back up before the stoppers are fully pressed home, so the sealed vial contains a low-pressure inert atmosphere rather than a hard vacuum.
Two consequences follow at the bench. The gas displaces oxygen, which matters for the three oxidation-prone residues identified in the oxidation post. And because the internal pressure is below atmospheric, a vial will often draw solvent inward slightly when it is first entered, and conversely a vial that has been pressurised by adding solvent without removing gas will push back. Letting a vial equalise rather than forcing it is the whole technique, and it is the reason solvent is added slowly down the vial wall rather than jetted into the cake.
The partial vacuum is also a weak integrity signal. A vial that offers no resistance at all on first entry, and never has, may simply have equalised through a compromised seal at some point. It is not proof of anything and it is worth noticing.
Coring
This is the failure mode that deserves a name.
When a needle passes through a stopper it should part the rubber, which then closes behind it. Coring is when it instead punches out a fragment, a small plug of rubber that falls into the vial or lodges in the needle. Two things go wrong at once. There is now a particle in the solution, and the stopper has a channel through it that will not reseal, so the vial's barrier is gone for every subsequent use and in storage.
What causes it is mostly geometry. Entering at a shallow angle makes the bevel shear sideways through the rubber rather than parting it. Entering repeatedly through the same spot thins and weakens that spot. A blunted or reused needle tears instead of cutting. Large gauges remove more material than small ones.
What avoids it is unglamorous. Enter through the centre of the stopper, close to perpendicular, with a sharp point, once per entry, and vary the entry point slightly across uses rather than tracking the same hole. If a fragment does end up in the solution, the solution contains a visible particle and should be treated as compromised rather than filtered and used, because the barrier is also gone.
Why entries are the limit
A reconstituted vial has two clocks running. One is time, which is the subject of how long a reconstituted peptide lasts. The other is entries, and it is the one people do not count.
Every entry does three things. It exchanges some headspace gas for room air, bringing in oxygen and water vapour. It carries whatever is on the stopper's face through the barrier, which is why that face is wiped and allowed to dry rather than wiped and immediately pierced. And it incurs a small, cumulative probability of coring.
For a dry vial the water exchange matters most, because a lyophilized cake is hygroscopic and the water it takes up is a reactant for most of its degradation routes, which residual moisture and which residues degrade first cover between them. Whatever the drying cycle achieved, a leaking or cored stopper undoes over weeks.
The practical response is not to be precious about it but to design around it. Withdraw into aliquots in one session rather than returning to the vial repeatedly, which is the argument of aliquoting and freeze-thaw, and keep a count of entries on the vial alongside the date it was reconstituted.
What the closure cannot do
It cannot make a solution sterile. A stopper that has been pierced has been crossed, and a closure maintains a barrier rather than restoring one. If sterility is a requirement of the experiment it comes from filter sterilising the solution, not from the vial.
It cannot keep a solution cold, dark or still, and it cannot slow any reaction happening inside. And it cannot tell you whether it has failed: a stopper with a channel through it looks much like one without, which is why entry technique is preventative rather than something you verify afterwards.
Frequently asked questions
What does the stopper colour mean?
It marks the rubber formulation the manufacturer used, which differs by filler and curing system. It is not a quality grade, and nothing can be inferred about a vial from the colour of its stopper alone.
Is the vial under vacuum?
Usually not a hard vacuum. The common practice is to backfill with an inert gas before the stoppers are fully seated, so the headspace sits somewhat below atmospheric pressure. That is why a vial often draws solvent in slightly on first entry.
Do I remove the metal collar?
No. The flip-off plastic cap comes away to expose the stopper's face; the crimped aluminium collar stays and is what holds the stopper in compression. Prising the collar off opens the container.
What is coring and how do I avoid it?
It is a fragment of stopper being punched out instead of parted, which contaminates the solution and leaves a channel that will not reseal. Entering through the centre, close to perpendicular, with a sharp point, and varying the entry point slightly between uses is what prevents it.
How many times can a vial be entered?
There is no number we can give for these vials, because that depends on closure data we do not hold. What is true is that each entry exchanges air and carries a small risk of coring, so the useful habit is to minimise entries by withdrawing into aliquots in one session and to record how many have been made.
Does the stopper keep the solution sterile?
No. Once pierced, the barrier has been crossed. A closure maintains a barrier; it does not create one, and sterility in a solution comes from filtration.
References
- United States Pharmacopeia. General Chapter <381> Elastomeric Closures for Injections, which sets out the material requirements and the coring test, and General Chapter <1207> Package Integrity Evaluation. www.usp.org
- ISO 8362-2:2015, Injection containers and accessories, Part 2: Closures for injection vials. The dimensional and material standard for the stoppers described here. www.iso.org/standard/61656.html
- Tang X, Pikal MJ. Design of freeze-drying processes for pharmaceuticals: practical advice. Pharmaceutical Research 2004;21(2):191-200. Stoppering and backfill at the end of a lyophilization cycle. doi.org/10.1023/b:pham.0000016234.73023.75
- Pepstral certificate library: sample descriptions across 114 certificates record the physical state of material as received, read 5 October 2026. pepstral.com/coa.html
Every product mentioned is sold for laboratory research use only and is not for human or animal use. Nothing on this page describes or recommends use of the material sold here in humans or animals.
